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Long execution time despite LIMIT 3
Posted by: Joachim Berger
Date: June 19, 2011 12:47PM

Hi,

it sounds stunningly basic, but I don't get it. We have a table containing some million messages (InnoDB), and an index on a field called 'thread'. Now, especially under heavy traffic, a simple

SELECT .. FROM table WHERE thread='foo' ORDER BY id DESC LIMIT 3

might run for more than 60 seconds(!!!!) when the result would return a large number of rows. But there's a LIMIT! So why does it take that long?

Regards,
Ben

Update: There's an index on (thread,id) being used, by the way.



Edited 1 time(s). Last edit at 06/19/2011 12:55PM by Joachim Berger.

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